Condition controlling apparatus



May 13, 1947. J. M. WILSON CONDITION CONTROLLING APPARATUS 2 Sheets-Sheet 1 Filed Aug. 25, 1944 o Q m N 3 n g m iilifimxx- 2% 5 k g as n m \IH A x Q. m 4/ on? u s {6% u a \Q @Q a Q Q\\| mm 8 u n Jo i/1V M. WILSON 4 1% By firm/inn May 13, 1947.

J. WILSON CONDITION CONTRCLLING APPARATUS Filed Aug. 25, 1944 2 Sheets-Sheet 2 W N Wm a Q\ S k Patented May 13, 1947 UNITED PATENT orncs CONDITION CONTROLLING APPARATUS John M. Wilson, Minneapolis, Minn., asslgnor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application August 25, 1944, Serial No. 551,226

23 Claims.

Broadly, an object of the present invention is to provide an improved condition controlling apparatus of the type employing an impedance ele- Other objects oi the invention will be apparent from a consideration of the accompanying speci fication, claims, and drawing, of, which ment responsive to a controlling condition and controlling the operation of an amplifier to in turn control the energization of a device which.

controls a condition changing device.

A further object of the present invention is to provide an arrangement of the type discussed in which an amplifier of relatively low gain is employed while at the same time maintaining a high degree of sensitivity.

A. further object of the present invention is to obtain a system such as set forth in the last object in which. a device controlled by the ampliher is effective as a result of the device being energized to change the value of the condition at which the device would otherwise be energized so as to increase the sensitivity 01' the apparatus.

A further object of the invention is to provide an improved burner control system of the electronic type in which the burner is placed into operation under the control oi an impedance responsive to a controlling condition and in which means responsive to the presence of the flame is eiiective to operate through an electronic amplifier to control the operation of the humor.

A further object of the invention is to provide a burner control apparatus of the type just described in which a thermionic amplifier associated with the impedance bridge is effective to prevent an undesired phase shifting which would otherwise occur in connection with the means :ior detecting a flame.

A still further object of the present invention is to provide an improved burner control system of the electronic type in which a safety switch is provided and in which an operating element of safety switch is initially connected in series with an electronic amplifier but upon the subsequent energization of a burner control device controlled by the amplifier is connected to a sep' arate source of power until combustion has occurred.

A still further object of the present invention is to provide a burner control system of the electronic type in which. a safety switch is provided and in which the operating element of the safety switch is connected in series with a cathode of the electronic portion of the apparatus.

Figure 1 is a schematic showing of a burner control system employing one form of my condition controlling apparatus, and

Figure 2 is a schematic showing of a burner control system employing a modified form of ml! condition controlling apparatus.

.Referring to the drawing for a more detailed understanding of my invention, the condition controlling apparatus is shown in Figure 1 as housed within a box ill. The apparatus is designed to control an oil burner Ii in accordance with the temperature conditions in the region at which a temperature responsive resistor i2 is located. The various elements will be described in more detail in the following paragraphs. Referring first to the condition controlling apparatus housed within'the box l0, this apparatus comprises two relays i3 and N. The relay i8 comprises a relay coil I5 and three switch blades l6, ii, and I8 which are adapted cooperate with contacts I9, 20, and 2i, respectively. The

switch blades i6, i1, and I! are normally biased out of engagement with contacts i9, 20, and 2! by any suitable means such as gravity. Upon energization oi the relay coil 05, the switch blades are moved into engagement with their respective contacts. Relay H similarly comprises a relay coil 24 and two switch blades 28 and 26 which cooperate with contacts 21 and 28, respectively.

' .Both switch blades 25 and 26 are biased into enrent supplied to these relays by the thermionic portion oi the apparatus. to be presently described.

The energizatlon oi relays i3 and H is controlled by a thermionic discharge device 32. This device is of the double triode type having two complete sets of tricdes and a common heater for the two cathodes. The various elements of the two trlodes are housed within an envelope 3;.

The first of the two triodes comprises an anode 3, a grid 35, and a cathode 36. The other of the two triodes comprises an anode 38, a grid 38, and a cathode M. The common cathode heater 41 is associated with both cathodes 38 and 48. Cathode heater 42 is connected to any suitable source of power, and in order to simplify the drawing, this source of power and the connections thereto have been omitted. The triode consisting of anode 34, grid 35, and cathode 88 controls the energization of relay I8 while the other triode controls the energization of relay I4 in a manner which will be more fully explained.

The triode including anode 34 is controlled in accordance with the output voltage of a bridge generally indicated by the reference numeral 45. This bridge comprises two fixed resistance elements 48 and 41, an adjustable resistance element 48, and a temperature responsive resistance element I2, previously referred to. The temperature responsive resistance element I2 is located outside of the box I8. preferably in a space whose temperature is to be regulated by operation of the burner II. One terminal of the temperature responsive resistance I2 is connected to a terminal 58 and the other to a terminal 5|. Terminal 58 is in turn connected to adjustable resistance 48 and terminal 5I by conductor 52 to resistor 41. The Junction of resistors 48 and 41 constitutes one of the input terminals of bridge 45 and terminal 58 the other input terminal, The Junetion of resistors 48 and 48 constitutes one of the output terminals and the Junction of resistor 41 and conductor 52 the other output terminal. Resistor I2 is of a material having a relatively high temperature coefficient of resistance, such as nickel, while resistors 48, 41 and 48 are of material having a negligible temperature coefllcient of resistance, such as manganin.

The resistance brid e 45 is connected to the input terminals of amplifier 82 by means including an electronic amplifier 54. This amplifier comprises an anode 5-5, a cathode 58, a control grid 51, a screen grid 58, and a suppressor grid 59. These various electrodes are housed within an envelope 88. The cathode 58 has associated therewith a cathode heater 8| in the conventional manner.

Power is supplied to the apparatus by a transformer 88. This transformer comprises a primary winding 84 and a plurality of secondary windings 85, 58, 81, and 88. The secondary 81 is center tapped at 89 and is provided with a further tap 18. While the values of the various other elements of the apparatus in one typical embodiment will be given later, it may be noted at this time that the primary winding 84 in this embodiment was a 110 volt winding. secondaries 85 and 88 were 38 volt and '1 volt windings, respectively. The winding 81 was a 600 volt winding, the tap 18 being spaced from the center tap 69 by about 6 volts. In the same embodiment, the winding 88 was a 20 volt winding.

The apparatus also includes a thermal safety switch 12. This safety switch may be of any suitable type in which upon continued energization of an electrically energized actuator, the switch is opened and requires a manual resetting operation to reclose it. A switch typical of the type which may be employed is that shown in the John M. Wilson Patent 2,290,399, issued Jul 21, 1942. As shown in schematic form in the drawing, the safety switch 12 includes a pair of switch arms 13 and 14 held in engagement with each other by a bimetallic element 15. Associated with the bimetallic element 18 is an electric heater 15. As long as the bimetallic element 18 is not heated beyond a certain temperature, it remains in a position in which the upper end of it lies beneath the switch blade 14 so as to maintain 4 switch blades 18 and 14 in contact making engagement. Upon continued energization of the electric heater 18, however, the bimetallic element 1-5 eventually warps to the left sufficiently that the left upper end thereof moves out of engagement with the switch blade 14 so as to permit switch blade 14 to drop away from switch blade 13. Thereafter, it is necessary for switch blade 14 to be manually reclosed before the circuit thereto can again be established. As will be pointed out later, where the apparatus functions in a normal manner, the circuit to heater 18 is interrupted before the switch blades 18 and 14 are separated.

The reference numeral 88 is employed to designate a diode comprising an anode M and a cathode 82. A cathode heater 1! is associated with the cathode 82 and is connected to secondary 88 in parallel with cathode heater 8| of tube 84. The diode 88 functions as a rectifier to supply a source of direct current power through the electronic amplifier of the apparatus. In this connection, it is to be noted that the anode II is connected by conductors 88 and 84 to the upper end or secondary 81. Cathode 82 is connected by a conductor 88 to a condenser 88 and a resistor 81. The condenser 88 and resistor 81. along with a further condenser 88, forms elements of a filter network. The Junction of resistor 81 and condenser 88 constitutes the positive terminal of this network and is connected by conductors 88 and 88, a resistor 8|, and a conductor 82 to the anode 55 of pentode 54. Condensers 88 and 88 are connected together by conductor 88 and to tap 18 or secondary 81 by conductors 81, 88, 88, I88, and NI, switch blades 18 and 14 of safety switch 12, conductor I82, heater 18, conductor I84, relay switch blade 25, contact 21, and conductor I85. The tap 18, it will be recalled. is spaced from the upper terminal of secondary 81 by a voltage of 306 volts. It will also be recalled that the anode 8i is connected to the upper terminal of secondary 81. Thus. the upper half of secondary 81 and the portion of secondary 81 between terminals 88 and 18 is connected in series with the diode 88 and the condensers 88 and 88. As previously noted, the upper terminal of condenser 88 constitutes the positive output terminal of the filter network. The lower terminal of condenser 88 constitutes the negative terminal being connected by conductors 81, I88, I88, and biasing resistor I88 to the cathode 58 of pentode 84.

The secondary 88 of transformer 88 is employed to supply alternating power to the resistance bridge 45. The upper terminal of this secondary is connected by conductor I81 to the input terminal 58. The lower terminal of secondary 85 is connected by conductor I88 to the junction of resistors 46 and 41 which, as previously noted, constitutes the other input terminal of resistance bridge 45.

The Junction of resistors 48 and 48, constitutin: one of the output terminals of the bridge 48, is connected by conductor II8, condenser III, and conductor II2 to the grid 51 of tube 84. The junction of conductor 52 and resistor 41, constituting the other output terminal of bridge 45.

is connected by conductors H4, H5, H8, and

biasing resistor I88 to the cathode 55. The outto grid 35 the tube 32.

the impedance of the output circuit of the bridge to match more nearly that of the input circuit of the pentode.

A condenser I20 is connected in parallel with the biasing resistor I06 and functions in the usual manner as a by-pass condenser to by-pass the alternating current component of the current flowing through the output circuit of tube 54.

The screen grid 58 of the tube is connected to the cathode 56 in the usual manner by a condenser I 22. It is connected to the anode 55 by a resistor I2 I, resistor BI and a by-pass condenser I23. and conductor 92. The suppressor grid 59 is directly connected to the cathode 56 in the normal manner.

As previously pointed out, the positive terminal of the filter network constituted by condensers 38 and 88 and resistor 81 is connected to anode 55, and the negative terminal of this filter network is connected to the cathode 56. Thus, a current flows through the output circuit of tube 54. which current is dependent in magnitude upon the input voltage applied by bridge 45.

The anode 55 of tube 54 is connected through conductors 92, I24, and I25 and condenser I26 Cathode 56 of tube 54 is in turn connected through biasing resistor I06 and conductors I03, I09, 98, and 99 to cathode 36 associated with grid 35. A resistor I2'I is connected between the cathode 36 and grid 35. An output voltage dependent in magnitude upon the conductivity of pentode 54 is thus impressed through condenser I26 between the grid 35 and cathode 36 of the lower triode of tube 32. This lower triode is accordingly controlled in accordance with the unbalance of resistance bridge 45. This lower triode in turn controls the energiza- While my invention is in no way limited to the use of elements having particular values, I have found it desirable in connection with one embodiment of my invention, to employ the following values for the various elements. In this embodiment, the transformer had the values previously noted. The resistors 46, 41, and I2 were each 500 ohm resistors while resistor 48 was a 480 ohm resistor. In this embodiment, the condenser III was a .02 microfarad condenser and resistor H8 2. 1 megohm resistor. The condenser I was a microfarad condenser while the resistor I06 was a 200 ohm resistor. The condenser I22 had a capacitance of .5 microfarad and resistor I 2I a resistance of .5 megohm. Resistor 8| had a resistance of 1 megohm and condenser I23 a capacitance of .01 microfarad. The condenser I26 had a capacitance of .1 microfarad and resistor I2! was a 5 megohm resistor. Tube 54 was a 6AC'7 tube and tube 32 a 7N7 tube. The resistors I31 and I38 were .25 megohm and 2.0 megohm resistors while the condenser I40 was a .02 microfarad condenser. The condenser I35 was a .02 microfarad condenser. In the power supply unit, resistor 81 was a 5,000 ohm resistor while condensers 86 and 81 each had a capacitance of 2 microfarads. A 20 Ohm resistor was employed for the safety switch heater I6.

OPERATION appreciably unbalanced so that an alternating tion of winding I5 of relay I3 as will be more voltage of substantial magnitude is impressed through condenser III upon the grid 51 of tube 64. This in turn causes a substantial flow of current between the anode 55 and cathode 56 through resistor 9|, the output circuit of tube 54 being from grid 39 through conductor I39, resistor I38,

conductor I42. condenser'I40, conductors I43 and IM, safety switch blades I3 and I4, conductor I45, switch blade I1. contact 20, and conductor I44 to the cathode 40.

As has been pointed out previously, the apparatus I0 is designed to control the operation of an oil burner I I in accordance with the temperature to which resistance bulb I2 is subjected. The oil burner II is of the conventional type being provided with a burner nozzle I through which is forced a mixture of fuel and oil by a blower driven by a motor I5I. Secured to the burner nozzle I50 or in any other suitable point is a, burner ignition means I52 which is operative when energized to supply an igniting spark to the fuel mixture. Located within the blast tube I50 or at some other suitable location is a photoelectric cell I54. This photoelectric cell is of any conventional type comprising an anode I 55 and a cathode I56.

Power for operation of the burner and for the primary winding 64 of transformer 63 is obtained from line wires I58 and I50. Primary 64 is connected to line wires I58 and I59 by a circuit as follows: from line wire I50 to conductor I60, line voltage terminal I6I, conductor I62, primary winding 64, conductor I63, conductor I65, line voltage terminal I66, and conductor I61 to line wire I58.

as follows: from the upper terminal of condenser (constituting the positive terminal of the power supply) through conductors 89 and 90, resistor 3|. conductor 02, anode 55, cathode 56, resistor I06, and conductors I03, I03, and 5'! to the lower terminal of condenser 88. The effect of this current flow is to impress across resistor 9I a voltage which is of such polarity that the lower terminal of resistor BI is negative with respect to the upper terminal. It will be noted that the grid 35 of the lower triode of tube 32 is connected through condenser I26 and conductor I25 to the lower terminal of resistor 9|. Thus, any increase in the magnitude of the voltage appearing across resistor 9I tends to drive grid 35 negative with respect to the cathode 36. Thus, with a temperature at or above the desired value, the degree of unbalance is such that grid 35 is maintained sufficiently negative with respect to the cathode 36 so as to prevent any appreciable flow of current through the lower triode of tube 32.

Now let it be assumed that the temperature of the medium to which resistor I2 is exposed drops so as to cause a decrease in the value of this resistor. The direction of unbalance of bridge 45 is such that upon a decrease in the value of resistance I2, this unbalance tend-s to be reduced. This in turn reduces the voltage impressed upon grid 51. which in turn decreases the voltage appearing across resistor 9|. This decrease in this voltage decreases the negative voltage applied to grid 35 of the lower triode of tube 32. When the temperature of the medium surrounding resistor I2 drops below the desired value, this decrease ,3 l potential of the grid 35 sufllciently to de the lower triode suiliciently conductive to ..e winding I of relay I3 through the 101- r; circuit: from the lower terminal of sec- El through conductor I18, relay winding do tor I1! and I12, anode 34, cathode 88, T iti, switch blades 18 and 14 tch 12, conductor I02, thermal safety T i151 lii, conductor I04, switch blade 25, met 21, and conductor I85 to the tap 18 of secondary 81.

The energization of relay I3 as a result of the ow through the last traced circuit or sufficient rent to energize relay I8 causes switch blades and I8 to move into engagement with is 1'8, 20, and 2I, respectively. The moveei switch blade I8 into engagement with act 2! causes the establishment of the 101- g energizing circuit to the burner motor from line wire I58, through conductor I81, oltage terminal I88, conductors I55 and "'t h blade I8, contact 2I, conductor I13, minal I14, conductor I15, burner motor onductor H6, burner terminal I11, conducto I18, line voltage terminal ISI, and con- ISO to the other line wire I58.-

the same time, a circuit will be established 12 ignition means I52 as follows: from line ough conductor I51, line voltage terconductors I85 and I18, switch blade .t H, conductor I88, switch blade 25, conductor I8I, ignition terminal I82,

Hi8, ignition means I52, conductors ":5, burner terminal I11, conductor I18, ltage terminal IGI, and conductor IGII to ier line wire I58. As a result of the enern of the burner motor and of the ignition the conditions necessary to the establish- U of combustion have now been set up.

movement of switch blade I6 into engageth contact i8 results in the establishor the following holding circuit for relay .5: from the lower terminal of secondary Jugh conductor I18, relay winding I5, conare ill and I12, anode 34, cathode 35, con- 88 and mi, switch blades 13 and 14, or MS, switch blade I6, contact I8, and i335 rid I85 to the center tap of the noted that this holding 5.. :JL ndent 'c not include the safety switch fety switch heater 16 is, howhy a circuit which will be next paragraph.

o relay switch blade I6 into ennth contact I8 also results in the folbeing established: from tap Cir i85, contact 21, switch blade 5 I84, electric safety switch heater I82, conductor I45, switch blade J? and, conductors I85 and I88 to 8 of secondary 61. As was noted,

"nit through winding I5 ining 18 of the safety switch. is the safety switch heater energizing circuit to relay ieater of the safety switch -oition before energlzation of r of the fact, however, that the ough an electronic tube is of titude, it is not possible .1 s of electric heater element, the current flowing through such a energize the safety switch heater.

: ltage applied to grid 85 is such as to of switch blade 25 and to energize the relay II.

For this reason, the apparatus is so designed as to provide for the establishment of the circuit last traced in which the heater 18 is connected independently of the tube 32 with the portion of secondary winding 81 between taps 68 and 10. This circuit insures a supply of sufficient current to electric heater element 18 to operate the saiety switch 12.

The engagement of switch blade I1 with contact 20 makes possible the establishment 01 a circuit through relay I4 through the following circuit: from the upper terminal of secondary 81 through conductors 84 and I85a, relay winding 24, conductor I86a, anode 38, cathode 40, conductor I, contact 28, switch blade I1, switch blade I6, contact I8, and conductors I85 and I88 to the center tap 88.

In spite of the establishment of the last traced circuit, relay Il will not be energized unless flame has been established at the burner II. In the absence of such a flame, condenser I35 is effective to bias grid 38 negatively to a point wherein insufllcient current flows through relay coil 24 This effect of condenser I85 is present because of the following condenser charging circuit: from the lower terminal of secondary 58, through conductor I88, condenser I85, conductor I38, resistors I31 and I38, conductor I38, grid 38, cathode l8, conductor I, contact 28, switch blade I1, switch blade I6, contact I8, conductor I85, and conductor I81 to the other terminal of secondary 58. Due to the rectifying eflfect oi the grid-cathode path, current tends to flow appreciably only in this one direction so that the right-hand terminal of condenser I35 becomes charged negatively with respect to the left-hand terminal. Since grid 38 is connected to the right-hand terminal of condenser I85, the effect of the connection is to maintain grid 38 at a potential negative with respect to the cathode 40.

Upon the establishment of a flame, the photoelectric cell I54, hitherto non-conductive, becomes conductive. As a result, a new charging circuit through condenser I35 is established. This charging circuit may be traced from the lower terminal of secondary 81, through conductor I18, relay winding I5, conductor I80, resistor I8I, conductor I82, photoelectric cell terminal I88, conductor I84, anode I55, cathode I58, conductor I85, photoelectric cell terminal I88, conductor I81, condenser I35, conductor I88, secondary 88, and conductors I81 and I86 to the center tap 58 or secondary 61. Thus, a voltage is impressed upon this charging circuit through the photoelectric cell, which voltage is equivalent to the voltage across the lower half of secondary 81 less the voltage of winding 88. As previously indicated, the voltage of the lower half of winding 81 is 300 volts while that of winding 68 is only 20 volts. Due to the rectifying effect of the photoelectric cell, current flows through this charging circuit only in the direction traced. It will be noted that the eilect of current flowing through the condenser I35 in this direction is to cause the righthand terminal or condenser I35 to become charged positively. It is this right-hand terminal oi conci nser I35 which is connected to the grid 88. the eiiect of the establishment of this chargi circuit as a result of the occurrence of flame to raise the potential of grid 38 with respect to cathode 48. This, in turn, renders the upper triode of tube 32 conductive so as to cause relay I4 to be effectively energized, This in turn results in switch blades 25 and 28 Q being moved out or engagement with contacts 21 and 23. The movement of switch blade 25 out of engagement with contact 21 interrupts the original energizing circuit for relay I3. This does not interfere with the operation of relay I3, however, in view of the previous establishment of the above traced holding circuit for relay I3 through switch blade I 6 and contact I3. The movement of switch blade 25 away from contact 21 also interrupts all circuits through the safety switch heater I5. This prevents the safety switch heater from causing the safety switch to move to open position and hence insure that the apparatus remains in operation.

The movement of switch blade 26 out of engagement with contact 23 interrupts the ignition means. This is desirable since with the establishment of combustion the operation of the ignition means is no longer necessary.

The apparatus is now in normal running condition and will continue to operate until the temperature of the medium in which the resistance bulb is located has risen above the desired value. When this happens, the unbalance of the bridge 45 will again be sufllcient to cause a biasing voltage to be impressed upon grid 35 such as to render the lower triode non-conductive. This in turn will cause the deenergization of relay winding I5, or, at least, reduce the energlzation to the point where the relay is acceptably deenergized. Deenergization of relay I3 results in the separation of switch blade I3 from contact 2| so as to cause deenergization of the burner motor. The separation of switch blade H from contact 20 results in the interruption of the energizing circuit to relay I I so that this relay likewise is immediately deenergized.

Operation in the event that flame is not initially established If a flame is not initially established, relay I4 remains deenergized with the result that switch blade 25 remains in engagement with contact 21.

As long as switch blade 25 is in engagement with contact 21, current continues to flow to the electrical heating element I5. After a predetermined time normally sufllcient for combustion to be established, the continued eflect of the energization with contacts 21 and 23. The reengagsment of switch blade 25 with contact 21 results in the reestablishment oi the energizing circuit for safety switch heater II. The reengagement of switch blade 28 with contact 23 results in the reestablishment of ignition. As a result, an attempt is made to reignite the burner. If the cause for extinguishment of combustion was merely temporary, the burner will be reignited and condenser I35 will again be charged positively to cause the reenergization of relay I4. If, however, combustion is not reestablished, the safety switch I2 will open at the end of the timing period. Thus, upon a failure of combustion, one attempt is made to restart the system.

Operation of flame detection apparatus in the event of accidental short circuiting of the photoelectric cell dentally bridged by a resistance, no matter how large or how small. Since the photoelectric cell is located in the blast tube, it is unlikely that any foreign substance will deposit on the electrodes, particularly a substance of a conductive nature.

However, if for this reason or any other reason,

the photoelectric cell terminals or leads are bridged by a resistance no matter how large or how small its magnitude, the apparatus will correctly respond in the absence of a flame, as though no flame is present. The reason for this is that the action of the apparatus depends upon the flow of a unidirectional current through the photoelectric cell. It is to be recalled that in tracing the charging circuit to condenser I35 which caused the right hand terminal of condenser I35 to become positive, it was pointed out that this flow of current took place in only one direction. If the photoelectric cell gap is bridged by an ordinary resistance, the flow of current through condenser I35 over this path will take place equally in both directions with the result that an alternating potential is impressed upon condenser I35. Further more, this alternating potential will be out of phase with the alternat ing potential impressed on anode 33 of the upper triode. This can readily be seen by the fact that the tendency of such a resistance is to establish a direct conductive connection from the lower end of secondary 61 through conductor I10, relay winding I5, conductor I90, resistance I9I,,con-

' ductor I92, photoelectric cell terminal I93, the

I5, I1, and I8 to move away from their respective contacts. This causes deenergization of the burner motor, which condition of deenerglzation will continue until switch blades I3 and I4 have been manually reclosed.

Operation in the event of the interruption of combustion after once established If combustion is interrupted after having once been established, the photoelectric cell I54 ceases to be illuminated with the result that it almost instantly becomes non-conductive. This in turn interrupts that one of the two charging circuits to condenser I35 which causes a positive charge to be impressed on the condenser. This in turn causes deenergization of relay It with the result that switch blades 25 and 25 are again reengaged bridging resistance, photoelectric cell terminal I 96, conductors I91, and I35, resistance I31 and I33 and conductor I39 to the grid 39. Since the lower end of secondary 51 is at a potential degrees opposed in phase with respect to the potential impressed upon anode 38, the efl'ect of this direct conductive connection is to cause the grid 33 to be highly negative during the conductive half cycle,

Were it not for the corrective phase shifting tendencies ofethe lower triode, the effect Just described would be offset by an undesirable phase shift which occurs by reason of condenser I35. It is to be noted that the grid 39 is in effect connected to the Junction of one terminal of a phase shifting bridge. One leg of this bridge consists of a condenser I35. The other leg of the bridge consists of the resistance across the photoelectric cell gap. These two legs are connected across the lower portion of secondary 51 and secondary 58. The cathode 40 is connected to the center 11 tap it when switch it is in enga eme t with contact II. Hence, the grid 80 is connected to a phase shifting bridge having as one leg the condenser lll and the other leg the resistance of the flame gap. The effect of such a connection, as the resistance across the photoelectric cell decreases more and more, is to progressively shift the phase of the alternating potential that is impressed upon grid 39. This would tend, under some conditions, to cause the alternating potential to be shifted so far as to actually render the triode conductive. This effect is more fully described in the copending application of Vilynn 0. Beam, Serial No. 450,613, filed Jul 11, 1942. As explained in that application, it was discovered that the presence of the second triode tended to correct this phase shifting tendency. Applicant has discovered that this phase shifting tendency of the second triode is present even when the second triode is employed as an amplifier for a separate signal source as in the present case in which the lower triode functions as the amplifier for the output of the bridge I! to control the energization of relay II. By reason of this discovery, it is possible to use only two triodes in one tube to perform three different functions. The upper triode functions as an amplifier for the detection of flame. The lower triode functions as an amplifier for the output of bridge 45 to control relay [3. Furthermore, the lower triode also functions as a phase corrective device in connection with the operation of the upper triode.

SPxcIEs or From: 2

The species of Figure 2 is similar to that of Figure 1 with the exception of the manner in which the safety switch is energized, the manner in which power is obtained for the amplifier 64, and in certain details of the amplifier circuit. In view of the similarity of many portions of the apparatus of Figure 2 to that of Figure 1, similar reference characters are employed in Figure 2 to designate those elements identical in character to the corresponding elements of Figure 1. To further aid in a comparison of the two figures, elements in Figure 2 which correspond to but differ in certain details from elements in Figure 1 have been given reference numerals 200 higher than the corresponding elements in Figure 1. All other elements of Figure 2 have been given reference numerals 300 or higher.

Relay 2i! corresponds to relay I3 of Figure 1. As is the case with relay i3, it comprises a relay winding 2|! which is connected in parallel with a by-pass condenser 230. Associated with the relay 2i3 are two switch arms 2" and Ill adapted to be moved into engagement with contacts 2H and 22!.

The relay 2 corresponds to relay ll of Figure 1. This relay comprises a relay winding 224 with which is connected in parallel a by-pass condenser 23L Associated with relay winding 224 are a pair of switch arms 225 and 226 designed to cooperate with contacts 221 and 226. Switch blades 225 and 226 are both biased into engagement with their associated contacts 221 and 226. Switch blade 226 further cooperates with a switch contact with which it is moved into engagement upon energization of relay winding 224. Relay 2 in addition comprises a third switch blade 302 which cooperates with a fixed contact 303. Switch blade 202 is biased out of engagement with fixed contact 303, being 12 moved into engagement therewith upon energization of relay winding 224,

In the present form of the apparatus, safety switch heater II is normally connected in series with the cathode heaters of tubes 22 and 64. In order to energize these heaters, the transformer 26! corresponding to transformer 62 of the species of Figure 1 is provided with an additional secondary winding 206. As will be described later, the connections of safety switch heater II to the secondary 306 are controlled by relay switch blade 226 and contacts 226 and Ill. When switch blade 226 is moved from engagement with contact 226 into engagement with contact 301 the safety switch heater I6 is disconnected from the cathode heater and in order to maintain the current flow to the cathode heater uniform, a resistor 306 is connected into the circuit in series with the cathode heater in place of the safety switch heater I6. This operation and the circuits connected therewith will be described in more detail in connection with the operation of the apparatus of Figure 2.

In the species of Figure 1, the upper portion of the secondary 61 was employed as a source of power for the rectifier 60. In the present species, a separate secondary Bill is provided for this purpose. The upper terminal of this secondary is connected by a conductor ill to the anode 8| of the diode 80. The lower terminal of secondary M0 is connected by conductor M2 to the lower terminal of condensers 66 and 66 of the filter network. The connections of the output terminals of the filter network to the anode and cathode of amplifier 254 are identical to those of Figure 1 and need not be repeated here.

A resistor 3I6 is connected between the cathode l0 and the center tap 66 by conductors ill, "6, 3H, I85, and I86. Upon energization of the relay 2i! as a result of a call for burner operation, this resistor H5 is shunted out so that the cathode 40 is connected directly to the center tap 69, as will be explained more fully later.

In order to render the apparatus more sensitive, a resistor 620 is under certain conditions connected in parallel with relay winding 2|. Resistor 320 is provided with a slider 22l for adiusting the respective value of the resistance.

In one embodiment of my invention in accordance with Figure 2 of the drawing, I found it desirable to employ for tube 56 a 1238 tube and for tube 22 a 14N7 tube. A safety switch heater 16 of approximately 23 ohms was employed and a resistor of the same resistance was employed for resistor 366. The resistor 320 connected in parallel with relay windin 2!! was a 10,000 ohm resistor and the resistor M6 was a 30,000 ohm resistor. In this embodiment. the various other elements had approximately the same values as in the embodiment referred to in connection with Figure 1,

OPERATION OI FIGURI 2 The operation of the lower triode of tube 22 which n controlled by the unbalance of the resistance bridge 45 is substantially the same as in the species of Figure 1 with one exception. In the arrangement of the present embodiment, the circuit to the cathode 36 includes the safety switch heater 16, the output circuit of the tube being as follows: from the lower terminal of secondary 61 through conductor llll, relay winding 2i6, conductor I12, anode 34, cathode 66, con- 13 ductor 336, conductor 333, safety switch heater 18, conductors 331, 338, 339, I86 and I88 back to the center tap 69 of secondary 61. It is to be noted that this circuit includes the safety switch heater 16. The reason for that is that by so including the safety switch heater in the initial energizing circuit for relay 2I5, it is impossible to energize relay 2 I3 to initiate operation of the burner and ignition means unless the safety switch heater is operative. This is similar to the arrangement of Figure 1 except for the location of the safety switch heater in the energizing circuit.

Except for this difference in the location of the safety switch heater, the operation of the resistance bridge, the amplifier 54, and the lower triode of the tube 32 is the same as in connection with' Figure 1.

Upon the energization of relay 2I3, switch blades 2I6, 2| 8, and 325 are moved into engagement with their associated contacts 2I9, HI, and 326. At the same time, switch blade 323 is moved out of engagement with contact 324. Moving of switch blade 2I6 into engagement with contact 2I9 completes a circuit through relay winding 224 as follows: from the upper terminal of secondary 61, through conductor I85, relay winding 224, conductor I86, anode 38, cathode 40, conductors I81 and 3, switch blade 2I6, contact M9, and

conductors 340, 3", I85 and I86 back to the center tap 69 of secondary 81. The establishment of this relay circuit does not, however, result in the energization of the relay until the photoelectric cell I54 is illuminated by flame. The operation of this portion of the apparatus will be described in a later paragraph.

The engagement of switch blade 2I8 with contact 22I establishes an'energizing circuit to the burner motor and to the ignition means, which energizing circuits are identical to those of Figure 1 and need not be again retraced.

As long as switch blade 323 was in engagement with contact 324, a circuit existed to the cathode heaters 42 and H as follows: from the upper terminal of secondary 306 through conductor 343, safety switch blades 13 and 14, conductor 344, resistor 308, conductors 345 and 346, switch blade 323, contact 324, conductors 348 and 849, cathode heater 6|, conductor 350, cathod heater 42, and conductor 35I back to the lower terminal of secondary 306. Upon the switch blade 323 moving out of engagement with switch blade 324, this energizing circuit for the cathode heaters is interrupted. Simultaneously, however, a new ener gizing circuit to the cathode heaters is established to the safety switch heater as follows: from the upper terminal of secondary 306 through con- (motor 343, safety switch blades 13 and 14, conductor 331, safety switch heater 16, conductors 336 and 354, contact 326, switch blade 325, conductor 355, relay contact 22 8, switch blade 226, conductors 356 and 349, cathode heater 6|, conductor 350, cathode heater 42, and conductor 35I back to the other terminal of secondary 308. It will be noted that the safety switch heater 16 is now connected in series with the cathode heaters.

The current flowing through the cathode heater is of a magnitude suilicient to adequately energize the safety switch heater 16. Because of the fact that the safety switch heater 16 is in series with the cathode heaters, a further check is made on the operation of the safety switch heater since if this heater is inoperative, the cathode heaters will become deenergized. Thus, upon switch blade 325 being moved into engagement with contact 326 as a result of the energization of relay 2I3, the safety switch heat r 16 start being heated to Initiate the timing cycle of safety switch.

Until the photoelectric cell I64 is subjected to a flame, the condenser I35 is charged negatively by reason of the following circuit: from the lower terminal of secondary 68 through condenser I35, conductor I36, resistors I31 and I38, conductor I39, grid 39, cathode 40, conductors I81 and 3I6, resistor 3I5, conductors 3I1, I85 and I81 to.the other terminal of secondary 68. The circuit corresponds to the circuit in Figure 1 for charging the condenser I35 negatively and the action thereof is exactly the same. Upon photoelectric cell I54 being illuminated as a result of flame, a charging circuit for condense I35 is established as follows: from the lower terminal of secondary 61 through conductor I10, relay winding 2 I5, conductors HI and I90, resistor I9I, conductor I92,

photoelectric cell terminal I93, conductor I94,

I35, secondary 68, and conductors I81 and I86 to the center terminal of secondary 61. Again, this circuit corresponds to th circuit in Figure 1 for charging the condenser I35 positively. As was the case in connection with Figure 1, the establishment of this circuit causes the potentialof grid 39 to be raised sufficiently to cause adequate energization of relay winding 224. This in turn causes movement of switch blades 302 and 226 into engagement with contacts 303 and 30I and movement of switch blades 225 and 226 out of engagement with contacts 221 and 228.

The movement of switch blade 225 out of engagement with contact 221 has the same effect as in Figure 1, that is, to deenergize the ignition means while leaving energized the burner motor.

The movement of switch blade 226 out of engagement with contact 228 and into engagement with contact 30I disconnects the safety switch heater from the cathode heater and again establishes a circuit for the cathode heater including resistor 308. This circuit may be traced as follows: from the upper terminal of secondary 306 through conductor 343, safety switch blades 13 and 14, conductor 344, resistor 308, conductor 345, contact 30I, switch blade 226, conductor 356,

conductor 349, cathode heater 6!, conductor 350,

cathode heater 42, and conductor 35I to the other terminal of secondary 306. Thus, before relay 2| 3 is energized, the cathode heaters are connected in series with resistor 30 8. After relay 2I3 is energized but before combustion has been established, the cathode heaters are connected in series with the safety switch heater so that the current flowing through the cathode heaters is effective to energize the safety switchheater. If combustion is established, th safety switch heater is disconnected and the fixed resistance 308 again connected in series with the cathode heaters. Since resistor 308 and safety switch heater 16 have substantially the same resistance values, the cathode heaters continue to be enerwinding 2|! is to decrease the current flowing through relay winding Ill. This has as its effect to reduce the amount which the current flowing through winding 2|! must drop in order tor the various switch blades to move to the position occupied when relay 2l3 is deenergized. The conventional relay has an appreciable differential between the current value at which it pulls in and the current value at which it drops out." The effect of resistance 320 is to materially reduce this differential. Because of this reduction in differential, the change in the value of resistance I2 exposed to the controlling temperature medium need be much less to cause relay 2! to move to deenergized position. This makes possible a much more sensitive system for a given amplifier than would be otherwise DOssible.

Any danger of chattering of the relay is eliminated by reason of the fact that the resistor 320 is not connected in parallel with relay winding 215 merely as a result of energization of that relay winding. This connection of resistor 320 does not occur until after combustion has been established so as to cause energization of relay 2. By this time, the conditions in connection with the energization of relay 2I3 are sufficiently stable that the connection of resistor 320 in parallel with the relay winding 215 thereof will not cause the relay to drop out unless the temperature affecting resistance element l2 has risen by the desired amount.

As soon as the temperature of element I2 has risen to the desired value, relay 2|! will drop out stopping operation of the burner. The resulting extinguishment of the flame will cause relay 2 to also be deenergized so as to completely shut down the system.

If combustion is not initially established, the safety switch heater IE will remain in series with the cathode heater until the safety switch opens. Since the safety switch is connected in series with the cathode heaters, this will result in demergization of the cathode heaters which in turn will cause both tubes 54 and 32 to become nonconductive. This in turn will cause deenergization of both relays 213 and 2|.

If, after combustion has been established, there is a flame failure, relay 2 will become deenergized so as to cause switch blade 225 to reengage contact 221 and switch blade 226 to move from engagement with contact I into engagement with contact 228. The reengagement of switch blade 225 with contact 221 will reenergize the ignition means so that an attempt will be made to reignite the burner flame. Movement of switch blade 22! from out of engagement with contact 3! into engagement with contact 228 will again connect the safety switch heater 16 in series with the cathode heater so that safety switch heater 18 will again be heated. If at the end of the timing period of the safety switch, combustion has not been reestablished, the safety switch will open and shut down the system in the manner previously described. If, on the other hand, the condition which produced the flame failure is a temporary one, combustion will be reestablished with the result that relay 2 will be reenergized. This will have the same effect as when this relay was energized in connection with normal operation.

The operation of the flame detecting mechanism is exactly the same in the event of short circuit or other abnormal conditions as in Figure 1 and need not again be described here.

Concwuox It will be seen in connection with both figures I have provided burner control apparatus in which the operation of the burner is controlled in accordance with the temperature of an impedance element having an appreciable temperature coefllcient of resistance and forming an element of an impedance bridge. The amplifying arrangement employed is extremely compact and functions to provide amplification both for the resistance bridge and also for the output of a photoelectric cell exposed to the burner flame. It will furthermore be seen that I have provided apparatus-by which the operation of the apparatus is carefully checked and the apparatus i shut down Whenever not functioning properly.

While I have described certain specific embodiments of my invention, it is to be understood that this is for purposes of illustration only and that my invention is to be limited only by the scope of the appended claims.

I claim as my invention:

1. In condition control apparatus, an electrically operated device for controlling operation of a condition changing means, said device being effective when energized at a first predetermined level of energization to move to an energized position and to remain in said position until the level of energization drops to a lower predetermined value, means including said device for causing operation of said condition changing means when said device is in its energized position, an impedance element separate from said condition changing means and having an appreclable temperature coefficient of impedance and subjected to a temperature condition indicative of the need for operation of said condition changing means, means operative upon the value of said temperature condition changing in one direction beyond a predetermined value to cause said device to be energized sufllciently to move to energized position, a further impedance element connected to said condition control apparatus, switching means for changing the connections of said further impedance element to said apparatus so as to change the degree of energization of said device for any value of said temperature condition, and means operative upon the establishment of a condition resulting from operation of said condition changing means to affect said last named means so as to decrease the amount of change of said temperature condition in the opposite direction necessary to cause said device to move from its energized position.

2. In condition control apparatus, an electrically operated device for controlling operation oi a condition changing means, said device being effective when energized at a first predetermined level of energization to move to an energized position and to remain in said position until the level of energization drops to a lower predetermined value, means including said device for causing operation of said condition changing means when said device is in its energized position, an impedance element having an appreciable temperature coefncient of impedance and subjected to a temperature condition indicative of the need for operation of said condition changing means, means operative upon the value of said temperature condition changing in one direction beyond a predetermined value to cause said device to be energized suiiiciently to move to energized position, a further impedance element, and means operative upon the establishment of a condition resulting from operation of said condition changing means to connect said further impedance element in parallel with said electrically operated device so as to decrease the amount of change of said temperature condition in the opposite direction necessary to cause said device to move from its energized position.

3. In burner control apparatus, an electrically operated device for controlling operation of a burner, said device being effective when enerized at a first predetermined level of energization to move to an energized position and to remain in said position until the level of energization drops to a lower predetermined value, means including said device for causing operation of said burner when said device is in its energized position, an impedance element having an appreciable temperature coefilcient of impedance and subjected to a temperature condition indicative of the need for operation of the burner, means operative upon the value of said temperature condition dropping below a predetermined value to cause said device to be energized sufiiciently to move to energized position, a further impedance element connected to said burner control apparatus, switching means for changing the connections of said further impedance element to said apparatus so as to change the degree of energization of said device for any value of said temperature condition, and means operative upon the establishment of combustion to operate said switching means in such a manner so as to decrease the value to which said temperature condition must rise to cause said device to move from its energized position.

4. In temperature control apparatus, an electrically operated device for controlling operation a temperature changing means, said device being efiective when energized at a-first predetermined level of energization to move to an energized position and to remain in said position until the level of energization drops to a lower predetermined value, means including said device ior causing operation or said temperature changing means when said device is in its energized position, a resistance bridge separate from,

said temperature changin means and including a resistance element having an appreciable temperature coeflicient of resistance and subjected to a temperature condition indicative of the need for operation of said temperature changing means, means responsive to the unbalance of said bridge and operative upon the value of said temperature condition changing in one direction beyond a predetermined value to cause said device to be energized sufilciently to move to energized position, and means operative as a result of the energization of said device to decrease the enersization of said device by a fixed predetermined amount so as to decrease the amount by which said temperature condition must change in the opposite direction to cause said device to move from its energized position. 1 5. In condition control apparatus, a relay 1'0 controlling a condition changing means and movable to and from an energized position, an impedance element having an appreciable temperature coefficient of impedance and subjected to a temperature condition indicative of the need fo operation of said condition changing means, means operative upon the value of said temperature condition changing in one direction beyond a predetermined value to cause said relay to be energized sufiiciently to cause it to move to its energized position, and means including a device responsive to a condition resulting from operation of said condition changing means for so affecting said last named means upon the establishment of said last named condition as to decrease by a fixed predetermined amount the extent of change of said temperature condition in the opposite direction necessary to cause said relay to move from its energized position.

6. In burner control apparatus, a burner relay movable to and from an energized position, an impedance element having an appreciable temperature coefficient of impedance and subjected to a temperature condition indicative of the need for operation of the burner, means operative upon the value of said temperature condition dropping below a predetermined value to cause said relay to be energized suificiently to cause it tomove to its energized position, and means operative as a result of energization of said relay for so aiIecting said last named means as to decrease by a fixed predetermined amount the extent to which said temperature condition must rise to cause said relay to move from its energized position.

7. In burner control apparatus, a burner relay, movable'to and from an energized position, an impedance element having an appreciable temperature coefllcient of impedance and subjected to a temperature condition indicative of the need for operation of the burner, means operative upon the value of said temperature condition dropping below a predetermined value to cause said relay to be energized sufficiently to cause it to move to its energized position, and means including a combustion responsive device for so aifectinfl said last named means upon the establishment of combustion as to decrease by a fixed predetermined amount the value to which said temperature condition must rise to cause said relay to move from its energized position.

8. In burner control apparatus. an electrically operated device tor controlling a burner, an electronic amplifier for controlling the energization of said device. condition responsive means for controlling said electronic amplifier, a safety cutout device including an electrically energized element and operative upon continued energization or said element to cause said safety cut-out degization of said element upon said combustion responsive device being subjected to combustion conditions.

9. In burner control apparatus, an electrically operated device for controlling a burner, an electronic amplifier for controlling the energization of said device, condition responsive means for controlling said electronic amplifier. a safety switch including an electrical heater and operative upon continued energization of said heater to open said switch, means including said safety switch for controlling said device, an energizing circuit for said device including said amplifier and said safety switch heater in series, means operative upon energization of said device to energize said heater independently of said amplifier, and means including a combustion responsive device for causing deenergization or said heater 19 upon said combustion reponsive device being subjected in combustion conditions.

10. In burner control apparatus, an electrically operated device for controlling a burner, an electronic amplifier for controlling the energization of said device, condition responsive means for controlling said electronic amplifier, a safety switch including an electrical heater and operative upon continued energization of said heater to open said switch, a source of power having first and second portions, an energizing circuit for said device controlled by said amplifier and including said device, said first portion of said source of power, said safety switch, and said safety switch heater, means operative upon energization of said device to connect said heater to said second portion of said source of power, and means including a' combustion responsive device for causing deenergization of said heater upon said combustion responsive device being subjected to combustion conditions.

11. In burner control apparatus, an electrically operated device for controlling a burner, an electronic amplifier for controlling the energization of said device, condition responsive means for controlling said electronic amplifier, a safety switch including an electrical heater and operative upon continued energization of said heater to open said switch, a normally closed switch, means including a combustion responsive device for opening said normally closed switch upon the establishment of combustion, a source of power having first and second portions, an energizing circuit for said device controlled by said amplifier and including said device, said first portion of said source of power, said normally closed switch, said safety switch, and said safety switch heater, and means operative upon energization of said device to establish a new circuit for said device including said first portion of said source of power but not said normally closed switch and to connect said heater to said second portion of said source of power under the control of said normally closed switch so that upon said combustion responsive device being subjected to combustion conditions, said device remains energized but said heater is deenergized.

12. In burner control apparatus, an electrically operated device for controlling a burner, a thermionic amplifier comprising ,an electron emissive cathode and an electric cathode heater therefor, a safety switch including an electric heater and operative upon continued energization of said heater to open said switch, means for applying an input voltage to said amplifier whose magnitude is dependent upon the demand for burner operation, means including said amplifier and said safety switch for controlling the energization of said device, a source of power, an energizing circuit for said cathode heater including said source of power and said safety switch heater, and means including a combustion responsive device for causing deenergization of said safety switch heater but not of said cathode heater upon said combustion responsive device being subjected to combustion conditions.

13. In burner control apparatus, an electrically operated device for controlling a burner, a thermionic amplifier comprising an electron emissive cathode and an electric cathode heater therefor, a safety switch including an electric heater and operative upon continued energization of said heater to open said switch, means for applying an input voltage to said amplifier whose magnitude is dependent upon the demand for burner opera tion, means including said amplifier and said safety switch for controlling the energization of said device, a source of power, an energizing circuit for said cathode heater including said source of power and said safety switch heater, a resistor having substantially the same resistance value as said safety switch heater, and means including a combustion responsive device operative upon said combustion responsive device bein subjected to combustion conditions to disconnect said safety switch heater from said energizing circuit and to substitute therefor said resistor so as to maintain continued energiz'ation of said cathode heater at substantially the same magnitude 14. In burner control apparatus, an electrically operated device for controlling a burner; a thermionic amplifier comprising an electron-emissive cathode and an electric cathode heater therefor, a safety switch including an electric heater and operative upon continued energization of said heater to open said switch, means for applying an input voltage to said amplifier whose magnitude is dependent upon the demand for burner operation, an energizing circuitfor said device including said amplifier, said safety switch heater, and said safety switch means operative upon the energization of said device to establish a separate energizing circuit for said safety switch heater including said cathode heater, and means including a combustion responsive device for causing deenergization of said safety switch heater but not of said cathode heater upon said combustion responsive device being subjected to combustion conditions.

15. In burner control apparatus, an electrically operated device for controlling a burner, a thermionic amplifier comprising an electron emissive cathode and an electric cathode heater therefor, an energizing circuit for said cathode heater, a safety switch including an electric heater and operative upon continued energization of said heater to open said switch, means for applying an input voltage to said amplifier whose magnitude is dependent upon the demand for burner operation, means including said amplifier and said safety switch for causing the energization of said device, means operative upon the energization of said device to connect said safety switch heater in series with said cathode heater in said energizing circuit, and means including a combustion responsive device operative upon said combustion responsive device being subjected to combustion conditions to again disconnect said safety switch heater from said energizing circuit.

16. In burner control apparatus, an electrically operated device for controlling a burner. an electronic amplifier for controlling the energization of said device, condition responsive means for controlling said electronic amplifier to control in turn the energization of said device in accordance with the value of the condition to which said condition responsive means responds, a second electrically operated device also controlling the operation of said burner, a second electronic amplifier controlling the energization of said second electrically operated device, means including a, device adapted to be subjected to a flame of said burner for controlling said second amplifier to cause energization of said second electrically operated device only when said burner is ignited, said last-named means including connections tending to produce an undesirable phase shifting in said means, and connections between said first 21 named and said second electronic amplifier for causing said first named electronic amplifier to counteract said phase shifting tendencies.

17. In burner control apparatus, an electrically operated device for controlling a burner, an electronic amplifier for controlling the energization of said device, condition responsive means for controlling said electronic amplifier to control in turn the energization of said device in accordance with the value of the condition to which said condition responsive means responds, a second electrically operated device also controlling the operation of said burner, a second electronic amplifier controlling the energization of said second electrically operated device, a condenser operatively connected to said second amplifier and controlling its operation, means including a device adapted to be subjected to a fiame of said burner for charging said condenser in such a manner that said second amplifier causes energization of said second electrically operated device only when said burner is ignited, said condenser tending to produce an undesirable phase shifting in said last named means, and connections between said first named and said second electronic amplifier for causing said first named electronic amplifier to counteract said phase shifting tendencies.

18. In burnercontrol apparatus, an electrically operated device for controlling a burner, an electronic amplifier for controlling the energization of said device, said amplifier comprising an anode, cathode, and control element, condition responsive means for applying a variable voltage to the control element of said electronic amplifier to control in turn the energization of said device in accordance with the value of the condition to which said condition responsive means responds, a second electrically operated device also controlling the operation of said burner, a second electronic amplifier controlling the energization of said second electrically operated device and also comprising an anode, a cathode, and a control element, both of said amplifiers being located within a common enclosure and having a common heating element for the oathodes thereof, means including a device adapted to be subjected to a dame of said burner for applying a variable voltage to the control element of said second amplifier to cause energization of said second electrically operated device only when said burner is ignited, and means operative in the event combustion is not established within a predetermined period of time after said burner has been placed in operation to interrupt operation of said cathode heater element.

19. In combination, a relay adapted to control the operation of a temperature changing apparatus to cause the latter to increase its temperature changing effect when said relay is energized, an amplifier having input and output circuits, 9. source of power having a portion thereof connected to said output circuit, means including said output circuit for causing energization of said relay when a biasing voltage less than a predetermined value is applied to said input circuit, an impedance bridge including an impedance element having an appreciable temperature coefiicient of impedance and exposed to a temperature condition indicative of the need for operation of the temperature changing apparatus, means for applying a portion of said source of power to the input terminals, of said bridge to energize the same, means for connecting the output terminals or said bridge to the input circuit of said amplifier, the impedances of said bridge having such relative values that when said temperature condition is at a desired value the bridge is unbalanced in such a direction and to such an extent that the output thereof results in a biasing voltage of said predetermined value being applied to said input circuit and such that upon said temperature condition changing so as to require an increase in the temperature changing effect of said temperature changing apparatus, said output voltage is decreased.

20. In combination, a relay adapted to control operation of heating apparatus to cause the latter to increase its heating effect when said relay is energized, an amplifier having input and output circuits, a source of power having a portion thereof connected to said output circuit, means including said output circuit for causing energization of said relay when a biasing voltage less than a predetermined value is applied to said input circuit, an impedance bridge including an impedance element having an appreciable temperature coefficient of impedance and exposed to a temperature condition indicative of the need for operation of the temperature changing apparatus, means for applying a portion of said source of power to the input terminals of said bridge to energize the same, means for connecting the output terminals of said bridge to the input circuit or said amplifier, the impedances of said bridge having such relative values that when the value of said temperature condition is at a desired value the bridge is unbalanced in such a direction and to such an extent that the output thereof results in a biasing voltage of said predetermined value being applied to said input circuit and such that upon said temperature condition decreasing so as to require an increase in the heating eifect of said heating apparatus, said output voltage is decreased.

21. In combination, a relay adapted to control operation of a heating apparatus to cause the latter to increase its heating effect when said relay is energized, an amplifier having input and output circuits, an alternating source of power having a portion thereof connected to said output circuit, means including said output circuit for causing energization of said relay when an alternating biasing voltage out of phase with the voltage applied to said output circuit and less than a predetermined value is applied to said input circuit, a resistance bridge including a res'istance element havin an appreciable temperature coefllcient of resistance and exposed to a temperature condition indicative'of the need for operation of the temperature changing apparatus, means for applying a portion of said source of power to the input terminals of said bridge to energize the same, means for connectin the output terminals of said bridge to the input circuit oi! said amplifier, the resistors of said bridge having such relative values that when the value oi said temperature condition is at a desired of a condition changing means, said device beingeifcctive when energized at a first predetermined level of energization to move to an energized position and to remain in said position until the level of energization drops to a lower predetermined value, means including said device for causing operation of said condition changing means when said device is in its energized position, a resistance network separate from said condition changing means and including a resistance element variable in accordance with the value of a condition indicative of the need ior operation of said condition changing means, means responsive to the unbalance of said bridge and operative upon the value of said condition changing in one direction beyond a predetermined value to cause said device to be energized sumciently to move to energized position, and means operative as a result of the energization of said device to decrease the energization of said device by a fixed predetermined amount so as to decrease the amount by which said condition must change in the opposite direction to cause said device to move from its energized position.

23. In condition control apparatus, a relay for controlling a condition changing means and movable to and from an energized position, a variable impedance variable in accordance with the value of a condition indicative of the need (or operation or said condition changing means, means onerative upon the value of said condition changing in one direction beyond a predetermined value to cause said relay to be energized sufliciently to JOHN M. WILSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,290,091 Brown et a1 July 14, 1942 1,989,829 Specht Feb. 5, 1935 2,275,368 Krause Mar. 3, 1942 2,236,624 Littwin Apr. 1, 1941 Brown Dec. 9, 1941 

